Lua Variable, Table, For Loop Syntax

Question

Just saw this in the Lua self examples...

  -- Example 24   -- Printing tables.
  -- Simple way to print tables.

  a={1,2,3,4,"five","elephant", "mouse"}

  for i,v in pairs(a) do print(i,v) end


  -------- Output ------

  1       1
  2       2
  3       3
  4       4
  5       five
  6       elephant
  7       mouse

  Press 'Enter' key for next example

I haven't seen this syntax before, for i,v in pairs(a) do print(i,v) end

Where did the v come into existence ?

Does the word in cause it to exist ?

By the same token, where does the i come into existence ?

Is this a syntax designed for tables ?

Thanks for any explanation.

lua.org/manual/5.1/manual.html#2.4.5 and lua.org/manual/5.1/manual.html#pdf-pairs also lua-users.org/wiki/ForTutorial and lua-users.org/wiki/IteratorsTutorial — Etan Reisner
– Etan Reisner, Commented Mar 10, 2015 at 19:55

akaRem · Accepted Answer · 2015-03-10 21:44:11Z

3

pairs returns an iterator over all fields and their values

more exactly it's a function of table and previous seen index which returns pair of index and its value.

> t = {4,5,6}
> p = pairs(t)
> =p(t)
1   4
> =p(t,1)
2   5
> =p(t,2)
3   6

there are 2 options: iterate over every keys or just those which are integers: pairs and ipairs functions

this loop is very similar to python's

l = [4,5,6]
for i, v in enumerate(l):
    ...

or

d = {"a":1, "b":2}
for k, v in d.iteritems():
    ...

if you know python (it looks like everyone knows it)

answered Mar 10, 2015 at 21:44

akaRem

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1 Comment

Tom Blodget Over a year ago

To prevent confusion: ipairs doesn't iterate over all keys that are integers; It iterates from 1 to n-1, where n is the index of the first nil value encountered.

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